ESA’s Herschel space observatory discovered water vapor around Ceres. This is the first unambiguous detection of water vapor around an object in the asteroid belt.

Ceres is the largest object in the asteroid belt, with a diameter of 950 km. Unlike most asteroids, though, it is spherical and belongs to a category of dwarf planets. It is thought that Ceres is layered with a rocky core and an icy outer mantle.

When the Solar System was first formed, it was too hot in its central regions for water to have condensed on any of the innermost planets. It is thought that water was delivered to Mercury, Venus, Earth, and Mars later on by asteroid and comet impacts. Thus, the presence of water vapor around Ceres is not an entirely unexpected surprise.

“This is the first time that water has been detected in the asteroid belt, and provides proof that Ceres has an icy surface and an atmosphere,” says Michael Küppers of ESA’s European Space Astronomy Centre in Spain, lead author of the paper published in Nature.

Herschel couldn’t make resolved image of Ceres, but astronomers derived the distribution of water sources on its surface by observing variations in the water signal during the dwarf planet’s rotation period.

“We estimate that approximately 6 kg of water vapor is being produced per second, requiring only a tiny fraction of Ceres to be covered by water ice, which links nicely to the two localized surface features we have observed,” says Laurence O’Rourke, Principal Investigator for the Herschel asteroid and comet observation program called MACH-11, and second author on the Nature paper.

The most direct explanation of water vapor production is through sublimation, where ice is warmed and directly transformed to gas. An alternate possibility is that geysers or icy volcanoes play a role in the dwarf planet’s activity.

More detailed information on Ceres is expected soon, as NASA’s Dawn mission is currently en route there for arrival in early 2015.